Exposure of cells to ultraviolet or ionizing radiation or to certain chemicals can lead to cell death, induction of mutations or malignant transformation. Ionizing radiation is of particular importance to the Health Science since it is at the same time a dangerous potential "pollutant" of our environment and an invaluable tool in therapeutic and diagnostic radiology. Our major aim is the identification of the damage induced by ionizing radiation at the heterocyclic DNA bases under in vivo condition and the characterization of the repair processes which accomplish the replacement of damage nucleotides in the DNA in bacterial and mammalian cells. We are planning to continue our studies of the incision-, excision- and strand resealing steps in prereplication excision repair of gamma- ray damaged thymine in E.coli. Excision of ring damaged thymine from gamma-irradiated bacteriophage DNA by crude extracts of wild type E.coli and selected E.coli "repair mutants" will be investigated. The individual repair steps will be further characterized by studies of the action of purified bacterial and mammalian "repair enzymes" on gamma- irradiated DNA. Excision repair of gamma-ray induced thymine damage will be investigated in mammalian cells using an isolated nuclei system. The repair capacity of nuclei from the following types of cells will be compared: Cancer- and normal cells: rodent- and human cells; rapidly dividing and non-dividing terminally differentiated cells; cells at different stages in the cell cycle. Complementation and reconstitution with purified repair enzymes will be attempted in case repair deficiencies are discovered. Excision repair of gamma-ray induced thymine damage and resealing of radiation induced DNA single strand breaks will be studied in skin fibroblasts from patients with Fanconi's Anemia and Bloom's Syndrome. These diseases are characterized by an increase in the occurrence of chromosomal aberrations and of leukemia and systemic cancer and could be due to a deficiency in chromosomal repair. Complementation and reconstitution experiments are planned in case repair deficiencies are discovered.